Peter, David - how would I go about having that difflock solenoid checked out?

Welcome Dirk

I think what you want to check is the diff lock function.

Put the car on loose ground engage 4L and switch the diff lock on.
The diff lock light should initially flash and when diff lock engaged it should be indicating on the dashboard.
Wind the window down ... the rear tyres should "bite" the ground when turning.
That means your diff lock is working. (+ your diff lock selonoid + all the other components in the diff lock system)

If its not doing as described above than there could be various components at fault of which the diff lock solenoid is the most common problem .